Abstract
The minimal promoter of the phosphotyrosyl phosphatase activator (PTPA) gene, encoding a regulator of protein phosphatase 2A contains two yin-yang 1 (YY1)-binding sites, positively regulating promoter activity. We now describe a role for p53 in the regulation of PTPA expression. Luciferase reporter assays in Saos-2 cells revealed that p53 could down-regulate PTPA promoter activity in a dose-dependent manner, whereas four different p53 mutants could not. The p53-responsive region mapped to the minimal promoter. Overexpression of YY1 reverses the repressive effect of p53, suggesting a functional antagonism between p53 and YY1. The latter does not involve competition for YY1 binding, but rather direct control of YY1 function. Inhibition of PTPA expression by endogenous p53 was demonstrated in UVB-irradiated HepG2 cells, both on the mRNA and protein level. Also basal PTPA levels are higher in p53-negative (Saos-2) versus p53-positive (HepG2, U2OS) cells, suggesting "latent" p53 can control PTPA expression as well. The higher PTPA levels in U2OS cells, programmed to overexpress constitutively a dominant-negative p53 mutant, corroborate this finding. Thus, PTPA expression is negatively regulated by p53 in normal conditions and in conditions where p53 is up-regulated, via an as yet unknown mechanism involving the negative control of YY1.
Highlights
The most frequent genetic alteration in human cancers is the mutational inactivation of the p53 tumor suppressor gene [1], indicating that loss of p53 activity plays an important role in human carcinogenesis
phosphotyrosyl phosphatase activator (PTPA) Expression Is Enhanced in U2OS Cells Constitutively Overexpressing the R273L Dominant-negative p53 Mutant—To further substantiate the former results and to check whether the differences in basal PTPA expression could be linked to the p53 status of the cells, we looked for ways to inhibit endogenous p53 expression and/or activity in U2OS cells, in order to examine their effects on basal PTPA expression
The data suggest that the p53 motifs are not required for the repression. Consistent with the former data, we could not obtain evidence for direct binding of p53 to different PTPA promoter-specific probes, encompassing p53(u) and/or p53(d), in gel retardation experiments. These results indicate that the suppressive effect of p53 on PTPA promoter activity is neither mediated by direct binding to the promoter, nor by direct competition with yin-yang 1 (YY1) binding to this promoter
Summary
The most frequent genetic alteration in human cancers is the mutational inactivation of the p53 tumor suppressor gene [1], indicating that loss of p53 activity plays an important role in human carcinogenesis. The observed inhibition of PTPA promoter activity by p53 seemed to be specific and did not result from a negative effect of p53 on transcription in general.
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